Climate commitment

Climate commitment studies attempt to assess the amount of future warming that is "committed" by a given level of climate forcings (usually the forcing from GHGs), even assuming no increase above the given level. The "given level" is often CO2 doubling or quadrupling; or the present level of forcing.

Basic idea

If a perturbation - such as an increase in greenhouse gases or solar activity - is applied to the climate system the response will not be immediate, principally because of the large heat capacity of the oceans.

As an analogue, consider the heating of a thin metal plate (by the sun or by a flame): the plate will warm more or less quickly. If a thick metal block is heated instead, it will take much longer until the entire block has reached equilibrium with the imposed heating because of its higher heat capacity.

Land only stores heat in the top few meters. Ocean water, by contrast, can move vertically and store heat within the ocean's depth (convection). This is why the land surface is observed to warm more than the oceans. It also explains the very large difference in response between

• "equilibrium" climate prediction runs in which only a shallow ocean is used and it is assumed that the climate has come to equilibrium and
• "transient" climate prediction runs (in which a full ocean is used) and the climate is out of balance.

Models

Main article: global climate model

Recent models forecast that even in the unlikely event of greenhouse gases stabilising at present levels, the earth would warm by an additional 0.5 °C by 2100, a similar rise in temperature to that seen during the 20th century. As ocean waters expand in response to this warming, global sea levels would mount by about 10 centimetres during that time. These models does not account for ice cap and glacier melting; a better estimate might be double or triple this value [1].

History

This is by no means a new idea [2]; the concept is discussed in the IPCC TAR [3] and in the SAR in 1995. At the time of the TAR there were not yet studies of the levels of unrealized climate commitment that might remain in the current climate.

External links

• http://www.nature.com/news/2005/050314/full/050314-13.html
• GRL, VOL. 28, NO. 8, PAGES 1535–1538, 2001, Committed warming and its implications for climate change, Richard T. Wetherald et al.

References

• Meehl G. A., et al. Sciencexpress, 10.1126/science.1106663 (2005).
• Wigley T. M. L., et al. Sciencexpress, 110.1126/science.1103934 (2005).